The invention is based on a brake booster as generally defined hereinafter. Brake boosters are already known (German Offenlegungsschrift No. 32 40 404), in which the brake valves connected to pressure supply apparatuses are built into displaceable booster pistons, so that coaxially aligned tappets provided for actuating the brake valves travel substantially equally long displacement distances as do the booster pistons. It is also possible to dispose a brake valve next to the associated booster piston (German Offenlegungsschrift No. 32 32 051), and to actuate it via a lever mechanism that is coupled with the booster piston and the coaxially aligned tappet. In this brake booster as well, the displacement paths of the booster piston are substantially equal in length to those of the tappet. The diameters of the master cylinder pistons, which are displaced by means of such brake boosters, are selected such that if the pressure supply apparatuses fail then the force of the operator's foot, which is exerted via the brake pedals, tappets and the booster pistons upon the master cylinder piston are adequate to generate sufficiently high emergency braking forces. This has the disadvantage, however, especially when used with disk brakes, of requiring very long pedal travel paths as well as greater length of foot movement or pedal forces.